1. Field of the Invention
The present invention relates generally to audio processing, and more particularly to techniques for expanding the speech bandwidth of an acoustic signal.
2. Description of Related Art
Various types of audio devices such as cellular phones, laptop computers and conferencing systems present an acoustic signal through one or more speakers, so that a person using the audio device can hear the acoustic signal. In a typical conversation, a far-end acoustic signal of a remote person speaking at the “far-end” is transmitted over a communication network to an audio device of a person listening at the “near-end.”
These communication networks often have bandwidth limitations that impact the speech quality of the acoustic signal when compared to other audio sources such as CD and DVD. For example, telephone networks typically limit the bandwidth of an acoustic signal to frequencies between 300 Hz and 3500 Hz, although speech may contain frequency components up to 10 kHz. As a result, speech transmitted using only this limited bandwidth sounds thin and dull due to the lack of low and high frequency components in the acoustic signal, which limits speech quality. In addition, this limited bandwidth can adversely impact the intelligibility of the speech, which can interfere with normal communication and is annoying.
Bandwidth expansion techniques can be used to reconstruct missing frequency components to artificially increase the bandwidth of the narrow band acoustic signal in an attempt to improve speech quality. Typically the missing frequency components are reconstructed by performing frequency folding, whereby the narrow-band acoustic signal is upsampled and filtered to form an expanded wide band acoustic signal.
A specific issue arising in bandwidth expansion concerns the bandwidth expansion of the noise within the acoustic signal. Specifically, since speech is typically a non-stationary signal which changes and contains pauses over time, the upsampling can also result in the bandwidth expansion of the noise present in the narrow band acoustic signal. This expansion of the noise is undesirable for a number of reasons. For example, the noise bandwidth expansion can result in audible artifacts which degrade the intelligibility of speech in the expanded wide band acoustic signal. In addition, in some instances the expansion of the noise may degrade the intelligibility of speech to below the intelligibility of the narrow band acoustic signal, which causes the speech quality to worsen rather than improve.
It is therefore desirable to provide systems and methods for expanding the speech bandwidth of an acoustic signal which can overcome or substantially alleviate problems associated with expanding the noise bandwidth.